ABSTRACT This project will assess the impact of primary care provider (PCP) payment policy on outpatient care, clinical events, and medical spending. This project focuses on dual-eligible Medicare-Medicaid beneficiaries, as they are among the most costly and clinically vulnerable individuals in both programs and have historically had suboptimal access to care. We will assess the effects of temporary increases in Medicaid payment rates for PCPs in 2013 and 2014 that were mandated by the Affordable Care Act (ACA); this policy increased PCP payments for dual-eligible beneficiaries by up to 25 percent. The intent of the policy was to improve access to primary care for low-income populations, especially given large expected increases in the number of individuals covered by private insurance and Medicaid coverage due to the ACA. While one study found increased PCP appointment availability associated with the higher payments, there is little evidence on the effects of the policy on actual utilization, clinical outcomes, or total spending. Despite the limited evidence, most states returned to pre-2013 Medicaid payment rates in 2015, while 16 states continued the payment increase. We will examine the association between PCP payment changes and three types of outcomes: 1) outpatient care, including PCP visits and sentinel quality measures; 2) clinical events, including emergency department visits, preventable hospitalizations, and mortality; and 3) total and component medical spending. Our primary analyses will use Medicare claims data for low-income beneficiaries (N=1.48 million in 2011) from 2011-2017 linked with supplemental data on providers and individual- and area-level characteristics. We will use a quasi-experimental difference-in-difference design with fixed effects (within-person) estimation approaches to compare (1) dual-eligibles with incomes ?100% of the federal poverty level (FPL) for whom PCP payments increased (policy intervention group) with (2) a concurrent control group of low-income (101-135% FPL) non-dual Medicare beneficiaries without payment changes who live in the same geographic areas. We will examine variation in outcomes for beneficiaries living in states that did and did not expand Medicaid, and did and did not continue the payment increase after 2014. We will also conduct supplemental analyses using the Massachusetts All-Payer Claims Database to examine outcomes for all Medicaid enrollees in the state (with and without Medicare). In short, we will use the natural experiment created by the ACA and comprehensive datasets to provide timely information on the impact of increasing provider payments for low- income patients, with a focus on vulnerable dual-eligibles. Information on the potential effects of this policy is urgently needed as the federal government and states are continuing to make ongoing decisions regarding provider payment policy.